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PiperOrigin-RevId: 194583126
Change-Id: Ica1d8821a90f74e7e745962d71801c598c652463

1 files changed, 145 insertions, 0 deletions

diff --git a/vdso/vdso_time.cc b/vdso/vdso_time.cc
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+// Copyright 2018 Google Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "vdso/vdso_time.h"
+
+#include <stdint.h>
+#include <sys/time.h>
+#include <time.h>
+
+#include "vdso/cycle_clock.h"
+#include "vdso/seqlock.h"
+#include "vdso/syscalls.h"
+
+// struct params defines the layout of the parameter page maintained by the
+// kernel (i.e., sentry).
+//
+// This is similar to the VVAR page maintained by the normal Linux kernel for
+// its VDSO, but it has a different layout.
+//
+// It must be kept in sync with VDSOParamPage in pkg/sentry/kernel/vdso.go.
+struct params {
+  uint64_t seq_count;
+
+  uint64_t monotonic_ready;
+  int64_t monotonic_base_cycles;
+  int64_t monotonic_base_ref;
+  uint64_t monotonic_frequency;
+
+  uint64_t realtime_ready;
+  int64_t realtime_base_cycles;
+  int64_t realtime_base_ref;
+  uint64_t realtime_frequency;
+};
+
+// Returns a pointer to the global parameter page.
+//
+// This page lives in the page just before the VDSO binary itself. The linker
+// defines _params as the page before the VDSO.
+//
+// Ideally, we'd simply declare _params as an extern struct params.
+// Unfortunately various combinations of old/new versions of gcc/clang and
+// gold/bfd struggle to generate references to such a global without generating
+// relocations.
+//
+// So instead, we use inline assembly with a construct that seems to have wide
+// compatibility across many toolchains.
+inline struct params* get_params() {
+  struct params* p = nullptr;
+  asm volatile("leaq _params(%%rip), %0" : "=r"(p) : :);
+  return p;
+}
+
+namespace vdso {
+
+const uint64_t kNsecsPerSec = 1000000000UL;
+
+inline struct timespec ns_to_timespec(uint64_t ns) {
+  struct timespec ts;
+  ts.tv_sec = ns / kNsecsPerSec;
+  ts.tv_nsec = ns % kNsecsPerSec;
+  return ts;
+}
+
+inline uint64_t cycles_to_ns(uint64_t frequency, uint64_t cycles) {
+  uint64_t mult = (kNsecsPerSec << 32) / frequency;
+  return ((unsigned __int128)cycles * mult) >> 32;
+}
+
+// ClockRealtime() is the VDSO implementation of clock_gettime(CLOCK_REALTIME).
+int ClockRealtime(struct timespec* ts) {
+  struct params* params = get_params();
+  uint64_t seq;
+  uint64_t ready;
+  int64_t base_ref;
+  int64_t base_cycles;
+  uint64_t frequency;
+  int64_t now_cycles;
+
+  do {
+    seq = read_seqcount_begin(&params->seq_count);
+    ready = params->realtime_ready;
+    base_ref = params->realtime_base_ref;
+    base_cycles = params->realtime_base_cycles;
+    frequency = params->realtime_frequency;
+    now_cycles = cycle_clock();
+  } while (read_seqcount_retry(&params->seq_count, seq));
+
+  if (!ready) {
+    // The sandbox kernel ensures that we won't compute a time later than this
+    // once the params are ready.
+    return sys_clock_gettime(CLOCK_REALTIME, ts);
+  }
+
+  int64_t delta_cycles =
+      (now_cycles < base_cycles) ? 0 : now_cycles - base_cycles;
+  int64_t now_ns = base_ref + cycles_to_ns(frequency, delta_cycles);
+  *ts = ns_to_timespec(now_ns);
+  return 0;
+}
+
+// ClockMonotonic() is the VDSO implementation of
+// clock_gettime(CLOCK_MONOTONIC).
+int ClockMonotonic(struct timespec* ts) {
+  struct params* params = get_params();
+  uint64_t seq;
+  uint64_t ready;
+  int64_t base_ref;
+  int64_t base_cycles;
+  uint64_t frequency;
+  int64_t now_cycles;
+
+  do {
+    seq = read_seqcount_begin(&params->seq_count);
+    ready = params->monotonic_ready;
+    base_ref = params->monotonic_base_ref;
+    base_cycles = params->monotonic_base_cycles;
+    frequency = params->monotonic_frequency;
+    now_cycles = cycle_clock();
+  } while (read_seqcount_retry(&params->seq_count, seq));
+
+  if (!ready) {
+    // The sandbox kernel ensures that we won't compute a time later than this
+    // once the params are ready.
+    return sys_clock_gettime(CLOCK_MONOTONIC, ts);
+  }
+
+  int64_t delta_cycles =
+      (now_cycles < base_cycles) ? 0 : now_cycles - base_cycles;
+  int64_t now_ns = base_ref + cycles_to_ns(frequency, delta_cycles);
+  *ts = ns_to_timespec(now_ns);
+  return 0;
+}
+
+}  // namespace vdso